The present invention generally relates to semiconductor processing, and in particular to a system for uniformly heating a photoresist.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and the film exposed with a radiation source (such as optical light, x-rays, or an electron beam) that illuminates selected areas of the surface through an intervening master template, the mask, forming a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Proper preparation of the photoresist is critical to obtaining extremely fine patterns after exposure of the photoresist. In a typical process, a few droplets of photoresist are applied to a spinning wafer. The photoresist is then xe2x80x9csoftbakedxe2x80x9d to remove solvent and anneal. The properties of the photoresist, and the quality of pattern transfer, are affected by the heating temperature and time. To achieve uniformity and quality of the photoresist layer, heating must be uniform and temperature must be accurately controlled.
Both the overall temperature history, and variations in the temperature history across the photoresist must be controlled. For example, baking time and temperature affect the photoresist layer thickness. While the layer thickness is typically in the range of 0.1 to 3.0 microns, variances in thickness should be kept less than +10-20 xc3x85 across the wafer. Small variations in the time/temperature history across the photoresist can substantially alter image sizes, resulting in lack of image line control. A uniform time/temperature history of the photoresist is especially important with chemically amplified photoresists because image size control may be drastically affected by only a few degrees difference in temperature. Often substantial line size deviations occur when the temperature is not maintained within 0.5 degree tolerance across a silicon wafer. For example, when a photoresist is baked onto a substrate (e.g., wafer), temperature tolerances of xc2x10.2xc2x0 C. are required.
Efficient systems and methods for uniformly and rapidly heating layers of temperature-sensitive film formed on semiconductor substrates are therefore desired to increase fidelity in image transfer.
The present invention provides a system that can be used to control photoresist baking temperature so as to facilitate uniform heating of a photoresist formed on a semiconductor substrate (e.g., wafer). The system includes a bakeplate on which a coated wafer can be placed; a plurality of lamps, and a plurality of optical fibers configured to direct radiation to various portions of the bakeplate. At least one lamp driving device is used to drive the lamps and at least one measuring device is used to measure a parameter indicative of temperature. In one aspect of the invention, the temperature is measured at a plurality of location on the bakeplate. In accordance with another aspect, the temperature is measured at a plurality of locations on a coated wafer, when such a wafer is placed on the bakeplate. A processor operatively coupled to the at least one measuring device and the at least one lamp driving system, is capable of receiving data from the at least one measuring device and is configured to control, at least partially based on such data, the at least one lamp driving device so as to regulate temperature at the plurality of locations where temperature is measured. Temperature may be measured based on reflected radiation; the temperature measuring device may be a spectrophotometer or an interferometer. The spectrophotometer may measure either absorptivity or color. It is preferred to use a spectrophotometer measuring absorptivity. When bakeplate temperature is measured, the bakeplate may include europium chelate.
In one aspect of the invention, the system is configured to monitor temperature of a coating on a wafer, when such a wafer is placed on a bakeplate, and to selectively drive a plurality of heaters so as to maintain the coating temperature at a desired level. Substantial uniformity in heating may thereby be achieved, increasing fidelity of image transfer. In another aspect, the system is configured to monitor and keep uniform the bakeplate temperature, which has the effect of maintaining a substantially uniform temperature of a coated wafer when placed on the bakeplate.
Another aspect of the present invention is a method, comprising the steps of placing a coated wafer on top of a bakeplate, heating a plurality of portions of the bakeplate; measuring a parameter indicative of the coating temperature at a plurality of locations on the coating, and independently controlling the heating of each of the bakeplate portions to regulate the coating temperature at each of the locations where temperature is measured.
A further aspect of the present invention is a method comprising the steps of placing a coated wafer on top of a bakeplate, heating a plurality of portions of the bakeplate; measuring a parameter indicative of temperature at a corresponding plurality of locations on the bakeplate, and independently controlling heating of each of the bakeplate portions to regulate bakeplate temperature at each of the corresponding locations where temperature is measured.